Droplet generation is a key stage in all droplet microfluidic systems. The most common methods of continuous flow droplet generation are T-junction and flow focusing. The latter is preferred in most systems for its faster generation speed and overall smaller droplet sizes. Flow focusing droplet generation, as shown in FIG. 1A, uses the intersection of two oil channels 100, 102 and one aqueous channel 104, to generate monodisperse aqueous droplets 106. Tuning the flow rate ratio between the oil and aqueous channels leads to droplet size modulation. As shown in FIG. 1B, for a mineral oil and water system, higher ratios lead to smaller droplets. This size modulation method, although simple and widely used, has a significant disadvantage: response speed. Using syringe pumps, flow rate stabilization following a change is on the order of seconds or even minutes. During the stabilization period, the generated droplet sizes change slowly as well.
Active methods are required for fast droplet size changes. The most common control method is electrical. Thermal, mechanical and magnetic control methods have also been demonstrated. Of these methods, magnetic control has seen the least development and impact. There are two main reasons for such limited impact. First, demonstrated methods use water-based ferrofluids as the discrete phase and mineral oil or silicone oil as the continuous phase. Ferrofluid droplets have limited use since the contents of the droplet are exposed to iron oxide nanoparticles. The presence of these nanoparticles can be detrimental for applications in cell biology or chemical reactions. The second limiting factor is the magnetic source. Demonstrated methods have used either permanent magnets or electromagnets to generate the magnetic field. Permanent magnets, although magnetically strong, do not provide ON/OFF switching capability. Obtaining multiple droplet sizes with a permanent magnet requires physically moving the magnet with respect of the generation region: a slow process with limited precision. Electromagnet-driven methods do provide ON/OFF switching capability, albeit at slower rates than electrical control. Also, conventional electromagnets are large, making them unsuited for complex and dense microfluidic architectures with multiple independent magnetic actuators.